Thiazolylketol acetates as glycosyl donors. Stereoselective synthesis of alpha-linked ketodisaccharides

TMSOTf-promoted glycosidation of 1-C-(2-thiazolyl)-alpha-D-galactopyranosyl acetate (alpha)-2 and 1-C-(2-thiazolyl)-alpha-D-mannofuranosyl acetate 7 donors with 1 equiv of primary 3 and secondary 5 sugar alcohols accepters gave exclusively the corresponding alpha-D-ketodisaccharides 4a, 8a, 11a, and 12a in 60-73% yield. On the other hand glycosidation of the 1-C-(2-thiazolyl)-alpha-D-glucopyranosyl acetate 6 with the primary alcohol 3 under the above conditions afforded a mixture of alpha- and beta-D-ketodisaccharides 9a and 10a in ca. 1:1 ratio. The important role of the thiazole ring for the easy glycosidation of these ketol acetates was pointed out by comparison with ketoses bearing a methyl, carboxymethyl, and 2-furyl group. Application of the thiazolyl-to-formyl deblocking reaction sequence to the thiazolylketodisaccharides gave the corresponding aldehydes which in turn were converted into alcohols and esters by reduction and oxidation, respectively.